Many oil wells experience high water production. High water production is undesirable because it necessitates artificial lift systems which must accommodate the volume of water produced as well as water handling facilities at surface. These requirements add significantly to the operational and capital expenditures associated with production. In some circumstances, the production of wells and fields have been suspended or abandoned with significant volumes of oil left in the ground as a result of the poor economics resulting from excessive water production.
Downhole oil/water separation ("DHOWS") systems have been developed to contend with increased water production. Such systems incorporate the use of downhole liquid/liquid cyclones to separate the oil from the water. The separated oil is lifted to surface and the water is reinjected downhole. Unfortunately, the application of DHOWS systems in these wells is complicated by the inherent production of solids, such as sand, with the oil. The solids that are produced tend to remain in the disposal water stream which is reinjected downhole. Depending on the solids volume and well conditions, the solids may invade and plug the disposal zone or they may accumulate in the well bore. In either case, the situation ultimately leads to a reduction in injectivity which reduces the effectiveness or precludes the further use of a DHOWS system.
Attempts have been made to remove sand from the disposal water stream. One such method is disclosed in International PCT Application No. GB96/02282 published as WO 97/12254 on Mar. 17, 1997. According to that method, a liquid/liquid cyclone may be used to separate the production fluid into an oil enriched stream and a water enriched stream. The oil enriched stream is transported to the surface and the water enriched stream proceeds to a solid/liquid cyclone where the water is separated from the sand and then transported to a downhole disposal site. While this method does provide a means for removing solids from production water before reinjecting it downhole, the presence of solids in the liquid/liquid cyclone causes significant erosion of that apparatus and the geometry and plumbing of the system have been found unfavourable. In addition, a system which removes solids downstream of the liquid/liquid cyclone cannot be easily incorporated into existing commercially available DHOWS systems.